1. Field of the Invention
The present invention relates to a wideband capable directional coupler.
2. Description of the Related Art
Directional couplers are used for detecting the levels of transmission/reception signals in transmission/reception circuits of wireless communication apparatuses such as cellular phones and wireless LAN communication apparatuses.
A directional coupler configured as follows is known as a conventional directional coupler. The directional coupler has an input port, an output port, a coupling port, a terminal port, a main line, and a subline. One end of the main line is connected to the input port, and the other end of the main line is connected to the output port. One end of the subline is connected to the coupling port, and the other end of the subline is connected to the terminal port. The main line and the subline are configured to be electromagnetically coupled to each other. The terminal port is grounded via a terminator having a resistance of 50Ω, for example. The input port receives a high frequency signal, and the output port outputs the same. The coupling port outputs a coupling signal having a power that depends on the power of the high frequency signal received at the input port.
Major parameters indicating the characteristics of directional couplers include insertion loss, coupling, isolation, directivity, and return loss at the coupling port. Definitions of these parameters will now be described. First, assume that the input port receives a high frequency signal of power P1. In this case, let P2 be the power of the signal output from the output port, P3 be the power of the signal output from the coupling port, and P4 be the power of the signal output from the terminal port. Assuming that the output port receives a high frequency signal of power P02, let P03 be the power of the signal output from the coupling port. Assuming that the coupling port receives a high frequency signal of power P5, let P6 be the power of the signal reflected at the coupling port. Further, let IL represent insertion loss, C represent coupling, I represent isolation, D represent directivity, and RL represent return loss at the coupling port. These parameters are defined by the following equations.IL=10 log(P2/P1)[dB]C=10 log(P3/P1)[dB]I=10 log(P03/P02)[dB]D=10 log(P4/P3)[dB]RL=10 log(P6/P5)[dB]
The coupling of the conventional directional coupler increases with increasing frequency of the high frequency signal received at the input port. The conventional directional coupler thus suffers from the problem that the frequency response of the coupling is not flat. Where coupling is denoted as −c (dB), an increase in coupling means a decrease in the value of c.
Mobile communication systems conforming to the Long Term Evolution (LTE) standard have become practically used in recent years, and further, practical use of mobile communication systems conforming to the LTE-Advanced standard, which is an evolution of the LTE standard, is under study. Carrier Aggregation (CA) is one of the key technologies of the LTE-Advanced standard. CA uses multiple carriers called component carriers simultaneously to enable wideband transmission.
A mobile communication apparatus operable under CA uses multiple frequency bands simultaneously. Accordingly, such a mobile communication apparatus requires a wideband capable directional coupler, that is, a directional coupler usable for multiple signals in multiple frequency bands.
JP 2014-057207A discloses a wideband capable directional coupler. In the directional coupler disclosed in JP 2014-057207A, the subline includes a first coupling section having strong coupling to the main line, a second coupling section having weak coupling to the main line and located closer to the coupling port than the first coupling section, a third coupling section having weak coupling to the main line and located closer to the isolation port (terminal port) than the first coupling section, a first non-coupling section not coupled to the main line, extending between the first and second coupling sections and having a length of a quarter or more of a wavelength corresponding to the service frequency band, and a second non-coupling section not coupled to the main line, extending between the first and third coupling sections and having a length of a quarter or more of the wavelength corresponding to the service frequency band.
For the directional coupler disclosed in JP 2014-057207A, one attenuation pole occurs in the frequency response of the coupling. This allows a reduction in a change in coupling in response to a change in frequency over a somewhat wide frequency band. According to the frequency response of the coupling of this directional coupler, however, the coupling increases with increasing frequency in a frequency band higher than the frequency at which the attenuation pole occurs. It is thus difficult for this directional coupler to reduce a change in coupling in response to a change in frequency over a wider frequency band.